Active thermographic methods are often used to detect subsurface defects in a test specimen; that is, defects that are not readily ascertainable by viewing the specimen's surface. Active thermographic methods are often preferred because they are non-destructive and because they are capable of quickly locating subsurface defects over a large surface area. These methods usually involve heating the surface of the specimen and monitoring the subsequent heat signature radiated over a period of time from the specimen by way of an infrared camera. Subsurface air gaps or vacuums within the tested specimen are good thermal insulators when compared with the surrounding material and will therefore appear as a high-contrast thermal discontinuity in the thermographic image sequence due to the differences in heat flow between the defect and the surrounding defect-free area.
In some cases, however, the subsurface defect does not appear clearly in the thermographic image sequence because the walls of the defect are in mechanical contact, allowing at least some heat flow across the defect. This type of defect is often called a “kissing unbond” defect and is illustrated in FIG. 1A. As can be seen in FIG. 1A, the upper 106 and lower 108 walls of the defect 100 touch each other. Conventional active thermographic methods often cannot detect this type of defect because the mechanical contact between the walls of the defect provides partial thermal conduction rather than a large thermal discontinuity, thereby decreasing the thermal contrast in the thermographic images. This sometimes occurs in bonded or laminated structures, where unbonded, partially bonded, or delaminated areas in the joint may appear completely bonded in the thermographic image sequence.
There is a need for a device and method that can detect a kissing unbond subsurface defect in a non-destructive manner via active thermographic techniques.